Cutting and polishing of gemstones, including diamonds, other precious stones and semi-precious stones is well known. Gemstone working apparatus typically takes the form of a rotatable disk, known as a scaife or a lap, which has a planar working surface in which is embedded an abrasive of a preselected grade. A preferred abrasive is diamond dust.
Initial formation or `cutting` of facets on a gemstone is performed by use of a relatively coarse abrasive, while fine finishing or `polishing` is performed by use of a relatively fine abrasive.
As the operations of cutting and polishing require different grades of abrasive, it is known to provide two separate working surfaces. Each working surface has embedded therein an abrasive of a preselected grade and is located at a separate work station. Accordingly, after all the facets have first been cut on a gemstone at a first work station employing a relatively coarse abrasive, the gemstone may then be transferred to a second work station employing a finer abrasive for polishing.
It is, however, also known to provide two different grade abrasives on a single working surface. In this arrangement, an abrasive of one grade is provided on a first, circular area of the working surface, and an abrasive of a different grade is provided over a second, generally ring-like area of the working surface, concentric with the first area.
The described dual arrangement has the advantage of enabling working of a gemstone with abrasives of two different grades at a single work station. However, since the two different abrasives wear at different rates, this advantage is lost once the finer abrasive has worn down.
Disclosed in U.S. Pat. No. 2,309,016 to Ryan is a composite grinding wheel, "particularly . . . adapted for grinding tools which have hard cutting tips mounted on a metal shank" (column 1, lines 2-4). Ryan describes a wheel which has "two separate abrasive bodies for simultaneously grinding a composite work piece made of two different materials (column 2, lines 10-14). The two abrasive bodies are an annular outer body and a circular inner body. The inner body fits into the outer body via a pair of cooperating screw threads, the height of the inner body thus being adjustable relative to that of the outer body, by rotation of the inner body relative to the outer body.
The inner body may be locked in position in any of a predetermined plurality of rotational positions relative to the outer body. These positions are governed by provision of a plurality of screw holes that are formed in an inner portion of the inner body, a corresponding plurality of screw holes formed in a base or "cup" body, and a plurality of screws each adapted for insertion into a pair of axially aligned screw holes. The inner body may be locked to the base by insertion of the screws through the screw holes through both the inner body screw holes and the base screw holes.
It will thus be appreciated that the ability to adjust the height of the inner body relative to that of the outer body is limited in accordance with the number of screw holes provided, and that it is almost impossible to achieve a uniform height working surface, whereby the working surfaces of both the inner and outer bodies are in coplanar registration.